Supramolecular assemblies of chiral propargylic alcohols

Article abstract of DOI:10.1002/anie.200601594

(Figure Presented) Six pack: A hexameric supramolecular structure with alternating enantiomers is formed in the solid state from propargylic alcohols. These hexamers further assemble to form channels and cages that host solvent molecules. The synergistic

Full text of DOI:10.1002/anie.200601594

Communications
Self-Assembly
DOI: 10.1002/anie.200601594
Supramolecular Assemblies of Chiral Propargylic
Alcohols**
Marilise Hyacinth, Maksymilian Chruszcz, Ki Sung Lee,
Michal Sabat,* Ge Gao, and Lin Pu*
Chiral propargylic alcohols are very useful synthetic precur-
sors to many organic compounds.[1] A few propargylic
alcohols were also found to form interesting supramolecular
structures through intermolecular interactions of the hydroxy
[2–4]
ꢀ
groups and the p electrons of the C C bonds.
Another
supramolecular interaction based on the quadrupole p–p
attraction between an aryl and a perfluoroaryl moiety was
used to create new materials for applications in solid-state
photopolymerization, liquid crystals, hydrogels, and so
forth.[5] We have initiated a program to combine the four
ꢀ
functional groups, including the aryl, perfluoroaryl, C C, and
OH groups, to make the diaryl-substituted chiral propargylic
alcohols and to explore their potential in supramolecular
chemistry. Herein, we present some of findings that indicate
that the presence of the functional groups not only makes the
self-assembly of propargylic alcohols possible but it also
allows for a finely tuned cooperation of interactions on a scale
rarely seen in other supramolecular systems. We show herein
that diaryl-substituted chiral propargylic alcohols can form
cyclic hexameric supramolecular assemblies as a result of the
À
cooperation between three major intermolecular forces: O
À
À
H···O hydrogen bonding, C H···F C hydrogen bonding
involving organic fluorine atoms, and p–p stacking interac-
tions between the pentafluorophenyl and phenyl rings. We
believe that this synergy of intermolecular forces could be
utilized in a variety of applications, including molecular
recognition and discrimination.
We synthesized the racemic propargylic alcohol rac-1
from the reaction of phenylacetylene with pentafluoroben-
zaldehyde in the presence of Et2Zn and HMPA at room
[*] M. Hyacinth, K. S. Lee, Dr. M. Sabat, Dr. G. Gao, Prof. L. Pu
Department of Chemistry
University of Virginia
Charlottesville, VA 22904-4319 (USA)
Fax: (+1)434-924-3170
E-mail: ms5c@virginia.edu
lp6n@virginia.edu
M. Chruszcz
Department of Molecular Physiology and Biological Physics
University of Virginia
Charlottesville, VA 22908 (USA)
[**] We thank Professor Wladek Minor (University of Virginia Medical
School) for helpful discussions and the Rigaku/MSC for loan of the
RAPID system. Partial support of this work from the National
Institute of Health (R01GM58454/R01EB002037) and from the HKL
Research Inc. (GI11496) is gratefully acknowledged.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Angewandte
Chemie
temperature (Scheme 1). X-ray analysis of the crystals of rac-
1 gave a hexameric supramolecular structure that contained
alternating RSRSRS enantiomers (Figure 1). The crystal
structure of rac-1 consists of centrosymmetric hexamers
Scheme 1. Synthesis of the propargylic alcohol rac-1. HMPA=hexamethyl
phosphoramide.
Figure 2. Cages in the crystal structure of rac-1.
and inorganic guests, such as glycerol, argon, small carbohy-
drates, and SF6. It is useful in the storage and transport of
volatile species.
In the same way as the preparation of rac-1, we also
synthesized the racemic compound rac-2 from the addition of
À
Figure 1. a) The network of O H···O hydrogen bonds, face-to-face p–p
À
stacking interactions, and C H···F hydrogen bonds that cooperatively
form the hexameric structure of rac-1. b) Channels formed along the
crystallographic a axis.
2-ethynylnaphthalene to perfluorobenzaldehyde. The crystal
structure of this compound shows the same hexameric
channel-like supramolecular structure as rac-1 (Figure 3).
The cages inside rac-2 are similar to those of rac-1
(Figure 2c). The diameters A and B are 7.76 and 8.46 Š for
each cage in rac-2, respectively.
formed by alternating R and S enantiomers (Figure 1). The
À
hexamers are held together by O H···O hydrogen bonds with
O···O donor–acceptor distances of 2.808, 2.771, and 2.777 Š.
The pentafluorophenyl and phenyl rings are almost exactly
parallel, with an average separation of 3.52 Š between the
ring planes. Additional stabilization comes from a network of
À
À
À
C H···F C hydrogen bonds between the ortho C H groups of
À
the phenyl ring and the ortho C F groups of the pentafluoro-
phenyl ring, with the C···F separations of 3.25, 3.36, and
3.27 Š comparable with analogous distances found in systems
with organic fluorine atoms.[6]
The hexameric structures of rac-1 stack with each other
along the unit-cell axis with the alternating parallel phenyl-
ethynyl–pentafluorophenyl p–p interaction to form infinite
channels. In a hexameric unit of rac-1, three R enantiomers
are up and three S enantiomers are down (Figure 2a). The
two stacking hexamers in a channel generate a cage that can
include guest molecules, such as dioxane and dichlorome-
thane. Figure 2b is the cross section of one channel in the
crystal structure of rac-1 and shows two cages, with each
including a dioxane molecule. Figure 2c gives the dimension
of two cages in rac-1. The diameters A
À
Figure 3. a) The network of O H···O hydrogen bonds, face-to-face p–p
À
stacking interactions, and C H···F hydrogen bonds that cooperatively
form the hexameric structure of rac-2. b) Channels formed along the
crystallographic a axis.
We synthesized the 1-naphthyl-substituted analogue rac-3
to explore the role of the C H···F C hydrogen bonds in the
supramolecular structures of rac-1 and rac-2. This compound
contains one less C H group ortho to
À
À
À
and B are 7.85 and 8.35 Š for each cage,
respectively.
the triple bond than rac-1 and rac-2,
The supramolecular assembly of rac-
and was thus expected to have less
À
À
1 is similar to that of Dianinꢀs com-
C H···F C hydrogen bonds. The X-
ray analysis of the single crystal of
rac-3 showed no hexameric structure.
pound.[7] Dianinꢀs compound was found
to be able to host a wide range of organic
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Communications
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Instead, this compound formed a centrosymmetric RRSS
À
tetrameric assembly, stabilized by a network of O H···O
À
À
hydrogen bonds in the solid state. No significant C H···F C
interactions were present in the crystal lattice of rac-3.
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À
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ture of rac-1 and rac-2. Its crystal
structure was similar to that of rac-3
with the centrosymmetric RRSS te-
trameric assembly. Thus, the study of
rac-3 and rac-4 demonstrates that the
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À
À
C F···H C hydrogen bonds are very
important for the hexameric supramolecular structures of rac-
1 and rac-2.
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difluorophenyl group in place of the pentafluorophenyl unit
in rac-1 and rac-2, to assess the importance of the p–p
attraction of the aryl–perfluorophenyl
system. The subsequent structural
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determination of rac-5 revealed a
linear assembly of the opposite enan-
À
tiomers held together by O H···O
hydrogen bonds rather than the hex-
À
americ structure of rac-1 and rac-2. Some weak C H···F
interactions were also found. However, no apparent stacking
forces were observed in this structure.
Our study of the various diaryl-substituted propargylic
alcohols demonstrates that the cooperation of three non-
covalent forces, O H···O hydrogen bonds, aryl–pentafluor-
À
À
obenzene p–p interactions, and C H···F hydrogen bonds, in
rac-1 and rac-2 is crucial for their supramolecular structures.
Decreasing any of the forces, as shown in the structures of rac-
3, rac-4, and rac-5, leads to a complete disruption of the
hexameric channel-like assembly. The supramolecular assem-
bly of rac-1 also exhibits promising host–guest interactions, as
evidenced by its complexation with molecules such as
À
dichloromethane and dioxane in the cages between the (O
H)6 planes. Further research is in progress to explore the
utility of these structures and functions. We are also preparing
the optically active analogues of these compounds by using a
catalytic asymmetric synthetic method to construct the
optically active supramolecular structures;[8] for example,
the combination of (R)-1 with (S)-2 could potentially
generate optically active crystals and chiral cages.
Received: April 23, 2006
Published online: July 17, 2006
Keywords: p–p interactions · fluorine · hydrogen bond ·
.
propargylic alcohols · supramolecular chemistry
[1] Selected examples: a) Modern Acetylene Chemistry (Eds.: P. J.
Stang, F. Diederich), VCH, Weinheim, 1995; b) J. A. Marshall,
5360
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